1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine in which a plasma generation region is formed around discharge electrodes of a spark plug.
2. Description of the Related Art
In the past, there has been known an ignition apparatus for an internal combustion engine which includes spark plugs each of which has its tip end portion presented in the interior of a corresponding cylinder of an internal combustion engine main body, a high voltage generation power supply that serves to apply a high voltage to the spark plugs, a magnetron that generates a microwave, and a microwave antenna that is mounted on each of the spark plugs, irradiates the microwave generated from the magnetron to the interior of a corresponding cylinder, thereby forming a plasma generation region around discharge electrodes of the spark plug (see, for example, a first patent document: Japanese patent application laid-open No. 2007-113570).
In this ignition apparatus, by irradiating a microwave to a mixture containing air and fuel mixed therein, the time of combustion of the fuel can be shortened to improve the combustion efficiency, thus making it possible to reduce the rate or amount of fuel consumed.
However, in the microwave oscillation device as described above, the magnetron is used which is a thermionic tube generally used for consumer products such as microwave ovens or the like.
The principle of operation of the magnetron is that when a magnetic field due to a magnet is applied to electrons coming out from a heated cathode filament to an anode or positive terminal, a Lorentz force is applied to the electrons to curve their trajectory, whereby the electrons thus curved resonate in a small chamber of the anode, which serves as a cavity resonator, thereby generating a microwave in a continuous manner.
However, in “International Telecommunication Union (ITU)”, the assignment of operation frequencies is defined in order to avoid electrical or radio interference and noise jamming between individual applications and uses. Industrial, Scientific and Medical (ISM) Use bands are designated for “dielectric heating”, and a frequency of 2,450 MHz is approved and allocated for microwave ovens according to the Radio Law in Japan.
Since in magnetrons for microwave ovens, the dimensions of an anode or positive terminal is set according to this approved microwave frequency, there is little or no room for reducing the sizes of magnetrons as a whole. Thus, the entire volume of each magnetron becomes about 10 cm (width)×about 10 cm (depth)×about 20 cm (height).
Accordingly, in an internal combustion engine main body composed of a plurality of cylinders, it is necessary to provide a magnetron for each cylinder that requires a large space for installation thereof, and hence there has been a problem that in actuality, it is difficult to install a plurality of such magnetrons on the internal combustion engine main body.
In addition, in case where such an ignition apparatus is installed on a vehicle, there has been another problem that it is difficult for the magnetrons of large weights to withstand or endure vibrations applied thereto.